Stop generating empty Mesh2D in canvas::Frame

wgpu/src/widget/canvas/frame.rs

@@ -262,6 +262,7 @@ impl Frame {
     ///
     /// [`Frame`]: struct.Frame.html
     pub fn into_primitive(mut self) -> Primitive {
+        if !self.buffers.indices.is_empty() {
             self.primitives.push(Primitive::Mesh2D {
                 origin: Point::ORIGIN,
                 buffers: triangle::Mesh2D {
@@ -269,6 +270,7 @@ impl Frame {
                     indices: self.buffers.indices,
                 },
             });
+        }
 
         Primitive::Group {
             primitives: self.primitives,